Auxiliary compensating antenna



y 1961 A1. HOPPER 2,984,183

AUXILIARY COMPENSATING ANTENNA Filed March 29, 1950 I rates 1assignments, to the United States of America as represented by theSecretary of the Army Filed Mar. -29,- -'1950,Ser. No. 152,689

3 Claims. (G1; 102 7012) My invention relates to electric proximityfuses utilizing the Doppler principle, and has particular reference toan arrangement for minimizing the effect of moisture on such a fuse.

One widely used circuit for proximity fuses employs a tuned diodevoltmeter across the antenna terminals, with the diode-antenna tuningcoil individually coupled to a stable oscillator operated at full powercapability. This arrangement requires an adjustable Vernier tuningcapacitance for individually resonating the diode-antenna circuit to thepower oscillator assembly. This design is very sensitive to changes inantenna reactance. Unless the diode circuit is tuned exactly, its sharpresponse makes it a frequency discriminator, resulting in spurioussignals for any small variation in triode capacitances. A small changein antenna capacity or oscillator frequency results in an appreciablechange of diode voltage in this type of fuse.

It has been noted that under high moisture conditions the diode voltagemay be lowered by as much as 50%. Under these conditions, by retuningthe diode circuit, the voltage can be brought back to normal. Thedifficulty is caused by moisture increasing the capacity between theantenna and ground. According to my invention, I neutralize thiscapacity change by adding another electrode, exposed to rain or moistureto a degree proportioned to the exposure of the antenna, and connectedto a point in the circuit such that the capacity effect above noted willbe neutralized. Such a point may be the high side of either the plate orthe grid coils, as will be more fully described below so that moisturewill change the frequency of the oscillator and the diode coil in aboutthe same degree so that the diode voltage will remain constant.

The specific nature of the invention, as well as other objects andadvantages thereof, will clearly appear from a description of apreferred embodiment as shown in the accompanying drawing in which:

Fig. 1 is a schematic general view of a fused projectile incorporatingmy invention,

Fig. 2 is a schematic circuit diagram showing the principle of myinvention and Fig. 3 is a detail of the circuit of the Fig. 2 showing amodification of my invention.

Referring to Fig. 1 projectile 1, which may be a bomb or rocket or anyother projectile, is provided with a proximity fuse 2, screwed into thenose thereof at 3 in conventional fashion. The antenna is a modifieddipole type wherein the ring member 6 comprises one of the radiatingmembers and conducting projectile body 1 with fuse body 2 comprises theother radiating member. The main fuse body 4 carries ring member 6spaced by suitable insulation from the main body 4 to provide a desiredrelation pattern, as is well known. Spaced between ring member 6 andfuse body 4 is an insulated supplemental element or neutralizing ring 7,the purpose of which will be more fully explained below. The fuse body 4ordinarily contains the arming device, electric 2,984,183 Patented May16, 1961 circuit, power supply, detector, and booster, as is well known,while the radio oscillator and other tube circuits are usually locatedas close to the forward end of the fuse as is practicable. However, theplacement of these elements is well known and forms no part of myinvention.

' pending on the physical construction and position of these"elementsand indicated schematically by the dotted'lines "at 9. There issimilarly an elfective capacitance between the ring member 6 and fusebody 2 as indicated at 11. The value of capacitance 11 affects theresonant frequency of the antenna-diode circuit which is establishedprimarily by diode coil 14 and adjustable condenser 15.

The modified dipole antenna radiates energy supplied by oscillator 12through plate coil 13 inductive to diode coil 14. The diode antennacircuit is tuned to the frequency of the oscillator and this tuning mustbe accurate to secure the desired sensitivity. Changes in the reflectedimpedance of the antenna due to the presence of a nearby reflecting bodyshow up in the diode circuit as apparent changes in antenna radiationresistance and thus affect the output of diode 17. When the projectileand reflecting target have a definite relative velocity toward eachother these changes appear as the well known Doppler frequency which isamplified and used to activate a thyratron to cause detonation in knownfashion. In order to compensate for the detuning of the antenna due tomoisture, 1 connect the neutralizing ring 7 to the high side of platecoil 13 as shown in Fig. 2. Alternatively i may connect neutralizingring 7 to the high side of grid coil 16 in which case the action issimilar except that the circuit parameters are different and must beadjusted accordingly. If the neutralizing ring 7 is correctlydimensioned and proportioned, it will be affected by extraneous moisturein the proper degree so that the oscillator frequency is altered by thesame amount and in the same sense as the resonant frequency of theantennadiode circuit. Thus there is no net change in frequency and aresonant antenna-diode circuit remains at its point of maximumsensitivity.

The moisture referred to may be moisture on the surface of the fuse dueto rain, or it may be merely a change in the moisture conditions of theatmosphere through which the fuse passes. The effect of moisture may beminimized by completing the encasing antenna in molded insulatingmaterial as described in U.S. patent application of Custer and Hopper,Serial No. 124,522 filed October 31, 1949, entitled Antenna forProximity Fuse." This expedient may be utilized together with thepresent invention to elfect practically complete neutralization of allextraneous moisture efiects.

It will be apparent that the embodiments shown are only exemplary andthat various modifications can be made in construction and arrangementwithin the scope of my invention as defined in the appended claims.

I claim:

1. An object detecting system comprising an antenna having tworelatively fixed radiating members insulated and spaced from each other,an oscillator coupled to said members, said oscillator setting up a highfrequency radiant energy field between said members, a detectoroperatively coupled to said members for detecting the effect ofreflecting objects in said field, and an exposed conducting element,said element positioned in said field and in fixed relationship to atleast one of said members and directly coupled to said oscillator, saidelement so coupled to said oscillator whereby a capacitance changebetween said radiating members due to the presence of a distributedamount of conducting matter in the vicinity of said mem- 3. hers iscompensated by an increasing capacitance change between said element andat least one of said members.

2. The invention according to claim 1 wherein said radiating memberscomprise a modified dipole antenna, an antenna-detector inputcircuitjsaid circuit coupling said detector with said antenna andcomprising the tuning means of said detector, said circuit beingresonant at the frequency of said oscillator, said element functioningto change the frequency of said oscillator by the same amount and in thesame sense as the tuning of the antenna-detector input is changed by thepresence of said 7 distributed conducting matter.

3. A proximity fuse for projectiles comprising an antenna having twofixed radiating elements insulated and spaced from each other, a highfrequency oscillator ineluding a thermionic tube having its outputcoupled to said antenna, a diode-detector, a resonant circuit for tuningsaid diode-detector to the frequency of said oscillator, an auxiliaryantenna element positioned intermediate the said elements and insulatedtherefrom, the radiating elements of said antenna directly coupled tosaid resonant circuit and said auxiliary antenna directly coupled tosaid high frequency oscillator whereby to compensate for capacitancechanges between said radiatin elements. I

References Cited in the file of this patent FOREIGN PATENTS 581,163Great Britain Oct. 3, 1946

